SF and Fantasy allow good authors and appreciative readers a chance to explore alternatives. A "thought experiment", if you will. Plato's Republic was speculative fiction when he wrote it; so was Thomas More's Utopia... and still is.

How many people would be familiar with basic libertarianism, had they not read Heinlein? Or a possibly workable model of anarchy, had they not read LeGuin?

And... as I have mentioned here before, possibly more than once: there was a time (early-to-mid '60s), when the only place outside of the library where you could find 1984, Brave New World and Atlas Shrugged was on the SF paperback racks with Farnham's Freehold.